The invention relates to a method for operating a combustion engine of a motor vehicle with multiple cylinders all of which are operated in a full engine operation, and part of which are switched off in a partial engine operation, wherein switching from the full engine operation to the partial engine operation only occurs when a partial engine operation torque which the combustion engine is capable to be provide in the partial engine operation is greater or equal to a target torque set at the combustion engine. The invention also relates to a corresponding combustion engine.
Methods of the aforementioned type are known from the state-of-the-art. They are used for operating the combustion engine which is usually mounted in a motor vehicle and has multiple cylinders, i.e., at least two cylinders. However, the combustion engine can of course also be used outside of a motor vehicle. All of the cylinders of the combustion engine are operated in the full engine operation, which means that each of the cylinders performs a complete working cycle of suction, compression, combustion and exhaust. In particular in an operating state of the combustion engine in which only a small torque is to be generated by the combustion engine and thus a partial load operation is given, high pumping losses or gas exchange losses occur. In order to reduce the latter the cylinders can be switched off in particular in the partial load operation, i.e., the combustion engine is operated in the partial engine operation. In the partial engine operation at least one of the cylinders is switched off which in particular means that no fuel is introduced into the cylinder and no combustion occurs in the cylinder. Hereby advantageously all valves of the switched-off cylinder are kept closed in order to lower the gas exchange losses and thereby reduce fuel consumption. Turning off the cylinder allows significantly reducing the consumption disadvantage in the partial load operation of a combustion engine, in particular an Otto motor, which is quantitatively controlled by derating, compared to a combustion engine in particular a Diesel motor or Otto motor with stratified charge.
Switching to the partial engine operation is usually only permitted when the torque the combustion engine is capable of providing in the partial engine operation, in the following referred to as partial engine operation torque, is equal to the target torque which is to be provided by the combustion engine. When switching from the full engine operation to the partial engine operation and vice versa the provided torque, i.e., the actual torque, has to be kept constant as exact as possible in a combustion engine with cylinder switch-off in order to prevent bucking of the motor vehicle which would unacceptably affect comfort. Because the at least one turned-off cylinder does not contribute to the torque, the still operating cylinders have to compensate the latter and have to provide an actual torque which corresponds to the target torque.
This means that in the partial engine operation the load point is raised for the still operated cylinders. This raising of the load point requires for example approximately doubling the air charge of the still operated cylinders when switching from the full engine operation to the partial engine operation—in which half of the cylinders are turned off—i.e., in case of a half-engine operation, and requires approximately halving the air charge when switching from the partial engine operation to the full engine operation. However, the speed at which this change of air charge can be performed is limited. For this reason, in the switching time period in which switching from the full engine operation into the partial mode operation or vice versa occurs for example by means of correspondingly adjusting the ignition time point, the torque provided by the combustion engine has to be controlled. In this way a smoothening of the course of the outputted torque over time can be achieved so that the bucking can at least be partially prevented. However, changing the ignition time point reduces the combustion efficiency of the operated cylinders and with this the overall efficiency of the combustion engine. Each switching between the full engine operation to the partial engine operation or vice versa is associated with a lowering of the combustion efficiency or the overall efficiency in particular due to the temporary change of the ignition time point. Therefore fuel consumption is increased at each turning on or turning off of cylinders. The switching time range is for example about 500 ms per switching direction. Within this time range the fuel consumption may double. On average, switching time ranges of about 300 ms result, in which the fuel consumption increases by about 50%.